1. Field of the Invention
The present invention relates to a solid-state image pickup device in which a large number of photoelectric conversion elements and scanning circuits for deriving optical information of the respective elements are integrated on a semiconductor substrate.
2. Description of the Prior Art
A solid-state image pickup device converts two-dimensional optical information into electric signals in time sequence, employing photoelectric conversion and scanning.
A solid-state image pickup device should have a resolution power equivalent to that of an image pickup tube used in current television cameras. Therefore, at least 500.times.500 photoelectric conversion elements in the form of a matrix, switches for selecting (X, Y) coordinates corresponding to the elements, an X (horizontal) scanning circuit and a Y (vertical) scanning circuit which turn the switches "on" and "off" and each of which consists of at least 500 stages are required. Accordingly, a solid-state image pickup device is ordinarily fabricated by the use of the MOS LSI technology which can realize a high integration density comparatively easily.
In the standard television format, in order to narrow the video bandwidth and to obtain a picture with only slight flickering, one field is subjected to the interlaced scanning every horizontal scanning line (in the vertical direction), and one picture (one frame) is formed of two fields.
As formats for interlaced scanning, there have been proposed (i) a format wherein every second horizontal scanning line is selected in each field and (ii) a format wherein every two horizontal scanning lines are selected in one of the combinations differing for the respective fields (refer to Japanese Laid Open Patent No. 51-57123). According to the result of experiments carried out by the present inventors, it has been determined, for the former format, that the period of time during which optical information of respective picutre elements are stored becomes one frame period, the sensitivity being high, but that since the charges of picture elements which are not scanned and read out in the first field are derived later in the next field, a large lag occurs in a picture image which has an area larger than one picture element. Usually, in the image pickup tube, the signal quantity of the third field with light intercepted is called the lag. The aforecited lag phenomenon, however, may be considered as the lag of the initial (first) field and can be termed a "lag induced by the scanning format". It offends the eye considerably.
The cause for the lag phenomenon is that, since light charges in substantially equal amounts are read out in the first and second fields, the first field lag becomes 100%. In the case of an image pickup tube, interlacing is effected by electron beam scanning. There is no guarantee that the light charges of the picture elements for the succeeding field are not read in the preceding field. Rather, considerable components of the charges are read as the diameter of the electron beam becomes comparatively large. For this reason, the lag of the first field does not become so great as 100%. In the solid-state image pickup device, the light charges of the picture elements for the next field remain unread, so that the lag of the first field amounts to 100%.
With respect to the latter interlaced scanning format in which pairs of horizontal scanning lines are selected, it has been revealed that the period of time during which the optical information is stored becomes one field period, the sensitivity lowering to one half the sensitivity of the former format, but that, since the lag can be erased, a very good picture is obtained.
This interlaced scanning format, however, has been proposed recently. Any of vertical scanning circuits actualizing the format has a complicated circuit arrangement and the number of constituent elements of each stage of the vertical scanning circuit becomes large, which forms a serious hindrance to the practical use of the solid-state image pickup device endowed with the scanning function based on the present format. More specifically, it is the interlaced scanning circuit (vertical scanning circuit) having a large number of constituent elements that determines the number of picture elements which can be arranged in the Y (vertical) direction. Enhancement of the resolution power cannot be achieved unless the number of constituent elements of the interlaced scanning circuit (vertical scanning circuit) is diminished to the utmost and, simultaneously, the density of integration of the picture elements to be arrayed is increased so as to lower the arrayal pitch intervals of the picture elements.